1. Technical Field
The present invention relates to a method of detecting X-rays for obtaining improved radiographic images ranging from about 10 keV to about 50 keV. The invention also relates to an apparatus for detection of incident radiation for radiographic imaging for applications ranging from about 10 keV to about 50 keV. Further, the invention relates to the use of such an apparatus for detecting incident radiation in scanned-slot medical imaging.
2. Background Information
In medical X-ray imaging, the central problem is achieving the best possible image at the lowest possible radiation dose. In order to accomplish this, high detection efficiency for all X-ray photons passing through the patient is crucial. Current X-ray imaging systems work with Detection Quantum Efficiencies (“DQE”) ranging from 10% to around 60%.
Silicon is in many ways the ideal detector material. Advantages of silicon include the high quality and purity of the crystal, and its very low cost due to research and development in the semiconductor industry and the large volumes of silicon used.
However, silicon is not advantageous for use in that its low atomic number corresponds to a decrease in stopping power for higher energy X-rays. A silicon detector wafer can be made with a maximum thickness of only around 1 mm, with the standard thickness about half of that. Thicker detectors require application of prohibitively high voltages to deplete the whole detector volume and become an efficient X-ray detector, if the X-rays are incident at a right angle to the surface. This corresponds to an efficiency of only 25% at 20 keV.
A solution to this problem is to orient the detector edge-on to the incident beam. In this geometry, the thickness of the silicon stopping the X-rays can be large enough to stop virtually all incident X-rays. This method is outlined in the invention described in U.S. Pat. No. 4,937,453 to Robert S. Nelson (“Nelson”). Edge-on detection for increased efficiency is also conceivable for other semiconductor detectors, but is particular important in the case of silicon because of the limited stopping power of this material due to its lightness.
A problem not anticipated in the method and device described in Nelson is that the semiconductor detector is typically mechanically damaged in a zone close to the edge when it is cut. The cutting is usually performed with a diamond saw or a laser. In this area, a large amount of background current is generated. The active sensors in the semiconductor wafer have to be placed some distance from the edge in order not to be saturated by this background current, which mask the signal from the X-rays. As a solution, the present invention includes one or several guard-rings that are placed between at least one of the active sensors and at least one of the edges in order to sink the current generated at the edge of the detector, thereby preventing it from reaching the active sensors. The distance between the edge and the active sensors are from 0.1 mm to 0.6 mm. X-rays stopping in this region will not be detected. This so-called dead area is equivalent to an inefficiency in the order of 20% in diagnostic X-ray imaging, such as mammography.
The loss of information is even more serious since the majority of the low energy photons that carry the most contrast information to the image will stop in the region close to the edge, which is not an active detector volume, while the high energy photons, with less contrast information, tend to penetrate further down into the detector.
According to DE 19 61 84 65, a detector array is taught having a number of detectors, each provided by a semiconductor plate of a directly converging semiconductor material with an electrode layer on two opposing sides. At least two detectors lie adjacent to one another, vertical to a surface receiving the incident radiation, with the main surfaces of the detectors set at an angle of between zero and 90 degrees to the latter surface. Separators of a radiation absorbing material are inserted between the detectors. The fundamental idea is to extend the length of the path of the incident radiation to the semiconductor detectors without increasing the distance between the electrodes. Moreover, the detector arrangement is intended for high energy radiation and is provided to detect Compton radiation.
JP 50732150 provides an arrangement for reducing manufacturing cost and improve SN, measurement precision, and using performance. An X-ray inspecting device is equipped with an X-ray source for irradiating X rays onto a sample in revolution, with a collimator having slits formed in radial form, in order to draw the X-ray which permeate the sample is arranged. A semiconductor detector for detecting the X-rays by a sensing part through the radiation of the X-ray beam from the slit is also provided. Since the X-ray inspection device is installed at an angle other than a nearly right angle to the plane parallel to the vertical direction revolution axis center of the semiconductor inspection part and the sample, and it is installed at an inclination of an angle incident to the extension line of the X-ray beam, the irradiation of the scattered X-ray due to the sample into the semiconductor detector is prevented, and the X-ray beam supplied from the slit can be irradiated on the whole surface or a part of the sensing part. Also, this arrangement is for high radiation applications.